RESEARCH Rapid River Incision Across An

RESEARCH

Rapid river incision across an inactive fault—Implications for patterns of erosion and deformation in the central Colorado Plateau

Joel Pederson1, Neil Burnside2,*, Zoe Shipton2,†, and Tammy Rittenour1 1DEPARTMENT OF GEOLOGY, UTAH STATE UNIVERSITY, LOGAN, UTAH 84322, USA 2DEPARTMENT OF GEOGRAPHICAL AND EARTH SCIENCE, UNIVERSITY OF GLASGOW, SCOTLAND G12 8QQ, UK

ABSTRACT

The Colorado Plateau presents a contrast between deep and seemingly recent erosion and apparently only mild late Cenozoic tectonic activity. Researchers have recently proposed multiple sources of epeirogenic uplift and intriguing patterns of differential incision, yet little or no quantitative constraints exist in the heart of the plateau to test these ideas. Here, we use both optically stimulated luminescence (OSL) and uranium-series dating to delimit the record of ﬂ uvial strath terraces at Crystal Geyser in southeastern Utah, where the Little Grand Wash fault crosses the Green River in the broad Mancos Shale badlands of the central plateau. Results indicate there has been no deformation of terraces or surface rupture of the fault in the past 100 k.y. The Green River, on the other hand, has incised at a relatively rapid pace of 45 cm/k.y. (450 m/m.y.) over that same time, following a regional pattern of focused incision in the “bull’s-eye” of the central plateau. The Little Grand Wash fault may have initiated during Early Tertiary Laramide tectonism, but it contrasts with related structures of the ancestral Paradox Basin that are presently active due to salt dissolution and focused differential erosion. We also hypothesize there may be a Pliocene component of fault slip in the region linked to broad-wavelength erosional unloading, domal rebound, and extension. An apparent rapid decrease in incision rates just upstream through Desolation Canyon suggests the Green River here may have recently experienced an upstream-migrating wave of incision.

LITHOSPHERE; v. 5; no. 5; p. 513–520; GSA Data Repository Item 2013319 | Published online 3 September 2013 doi: 10.1130/L282.1

INTRODUCTION Several sources of middle-late Cenozoic regional ancestral Paradox Basin. In the cases of the Gra- uplift have been recently proposed for the Colo- bens district of Canyonlands National Park in The Colorado Plateau of the western United rado Plateau. These include buoyancy modifi ca- southeastern Utah and the Onion Creek diapir States is famous for the spectacular erosional tions of the mantle lithosphere linked to an ances- to the north, it has been established that defor- exhumation of a stratigraphic record that has try involving the Farallon slab (e.g., Humphreys mation is ongoing today (e.g., Colman, 1983; been subject to only mild tectonic deformation et al., 2003; Roy et al., 2009), regionalized Huntoon, 1988; Furuya et al., 2007). It also has over Phanerozoic time. Overall erosion of the dynamic support from convecting asthenosphere been widely speculated that localized dip-slip region is linked to a pulse of late Cenozoic inci- and potential mantle drips (Moucha et al., 2009; faulting and E-W–oriented graben formation sion driven by the integration and base-level drop van Wijk et al., 2010; Levander et al., 2011), and continued into the Quaternary or is still active in of the Colorado River off the southwestern mar- isostatic rebound due to unloading by erosion the central plateau (Colman and Hawkins, 1985; gin of the plateau to the Gulf of California (Luc- and extension (Pederson et al., 2002; Roy et al., Doelling et al., 1988; Shipton et al., 2004). The chitta, 1972; Pederson et al., 2002). That plateau 2009; Karlstrom et al., 2012). Indeed, the fl ex- Little Grand Wash and Salt Wash (aka Ten- margin in the western Grand Canyon area is the ural feedback between late Cenozoic exhumation Mile) Graben faults crossing the Green River focus of scientifi c controversy because of its and rock uplift is focused upon the central pla- are examples that have been a focus of recent complex and long paleocanyon-cutting history teau, where more than 3 km of section have been work (Fig. 1A). These faults have acted as path- spanning the Cenozoic (cf. Polyak et al., 2008; removed in areas (Nuccio and Condon, 1996; ways for fl uid fl ow, resulting in a set of aban- Karlstrom et al., 2008; Wernicke, 2011; Flowers Pederson et al., 2002; Hoffman et al., 2011; Karl- doned and modern spring-travertine mounds and Farley, 2012). However, the southwest mar- strom et al., 2012). The patterns of incision and focused along the fault traces (Shipton et al., gin contrasts with the core of the Colorado Pla- their relation to these distinct potential sources of 2004; Dockrill and Shipton, 2010; Kampman teau physiographic province, including our study regional uplift and other controls are also highly et al., 2012). The travertine mounds themselves area, which has a notably younger and more debated (e.g., Karlstrom et al., 2012; Pederson have been incised, permitting detailed study active record of erosion and landscape evolution and Tressler, 2012; Darling et al., 2012; Pederson of their internal chronostratigraphy (Burnside (Hoffman et al., 2011; Pederson et al., 2013). et al., 2013), but there are few well-constrained et al., 2013), which may constrain Quaternary geomorphic records available in the heart of the movement on these structures. Yet, no solid geo- Colorado Plateau to address these problems. morphic constraints on the timing and rates of *Current address: School of Geosciences, Univer- An exception to the general lack of deforma- faulting have been reported for the region. sity of Edinburgh, Scotland EH9 3JW, UK. †Current address: Department of Civil and Environ- tion in the Colorado Plateau is the episodic salt River terraces are valuable markers for these mental Engineering, University of Strathclyde, Scot- tectonics in the central plateau linked to unload- tectonic geomorphology problems, enabling us land G4 0NG, UK. ing of Pennsylvanian evaporite deposits of the to quantify rates of erosion, faulting, and land-

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110 o 15’ W 110o W A mouth of Deso-Gray Canyon

Book Cliffs 39 N

town of o Green River

fault B sh 70 Wa B and Little Gr

Salt Valley graben

Salt Wash graben Green River

Green River KILOMETERS anticline 0 0.5 1 38 45’ N

CONTOUR INTERVAL 40 FEET o

Moab fault 4200 Labyrint KILOMETERS

Canyon 0 5 10

4400

Little Grand Wash fault Crystal Geyser 7 Figure 1. (A) Location of the Crystal Geyser study area in the north- 0 central Colorado Plateau of the western United States, near the 420 6 5 2 5 town of Green River in the plateau badlands downstream of the mouth of Desolation–Gray Canyon and the Book Cliffs and upstream of Labyrinth Canyon. (B) Generalized ﬂ uvial terrace levels (num- 5 4 bered) where the Green River crosses the Little Grand Wash fault. 3 1/Qal Detailed mapping results are presented in Data Repository item 1 (see text footnote 1).

scape evolution. The trunk drainages of the pla- incision at a location in the north-central Colo- nate the patterns of erosion and deformation in teau have locally preserved a sequence of grav- rado Plateau where such constraints are missing. this landscape. elly strath (thin sediment cover) and thick ﬁ ll We utilize the archive of Green River terrace terraces that record both incision and responses deposits and associated travertine near Crystal BACKGROUND to climate change (e.g., Marchetti and Cerling, Geyser, at the intersection of the Green River 2005; Pederson et al., 2006). Through strati- and the Little Grand Wash fault (Fig. 1B; DR1 Setting graphic and geochronologic study, these can be map1). Field and geochronology results at Crys- used to constrain rates of local faulting and also tal Geyser reveal clear evidence for active river The Crystal Geyser study area lies along the provide time-integrated rates of incision along incision, but not active faulting, helping illumi- Green River, 7 km south of the town of Green the trunk drainages that set the pace for broader 1GSA Data Repository Item 2013319, a 1:12,000 scale surﬁ cial geologic map of the study area (item 1) and erosion in the landscape. tables, graphs, and descriptions of luminescence methods and results (item 2), is available at www.geosociety The goal of this study is to document any .org/pubs/ft2013.htm, or on request from [email protected], Documents Secretary, GSA, P.O. Box 9140, late Quaternary faulting and the rate of river Boulder, CO 80301-9140, USA.

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River, Utah (Fig. 1A). Flowing to the south, the tine and fault relations, previous workers have past level of the main-stem Green River (DR1 river exits Desolation–Gray Canyon through the proposed some amount of late Pleistocene fault map [see footnote 1]; P6/5 on cross section in Book Cliffs upstream and crosses low-relief, movement on the Little Grand Wash fault (Wil- Fig. 2; Table 1). arid badlands underlain by the upper Creta- liams, 2004; Shipton et al., 2004).

ceous Mancos Shale before entering Labyrinth Crystal Geyser itself is a periodic, CO2- METHODS Canyon downstream of the study area. As the charged geyser created by an oil exploration river approaches the Little Grand Wash normal well drilled in A.D. 1935 (McKnight 1940). Surﬁ cial deposits and terraces along the

fault, the lower Cretaceous Cedar Mountain and Travertine precipitates out of the CO2-saturated main-stem Green River corridor in the area of the Jurassic Morrison and Summerville Forma- water exiting the well, and there is a 113-k.y.- Crystal Geyser have been mapped at a scale tions rise in the footwall, forming a shallow and long record of fracture-ﬁ ll and travertine for- of 1:12,000 (DR1 map [see footnote 1]). The short canyon. Where the river crosses into the mation along the central part of the fault trace heights of terrace treads (top surfaces) and hanging wall, the Mancos Shale is brought to (Burnside, 2010; Burnside et al., 2013). Previ- straths (basal unconformities) were recorded river level again, the valley broadens, and there ous work at the site has generally focused on the in survey transects normal to the river channel is preserved a suite of seven gravelly strath ter- Little Grand Wash fault and Salt Wash Graben using a real-time kinematic global positioning

races in an interior bend of the river (DR1 map as a conduit for CO2 and water, and the traver- system (GPS) system. Both main-stem and local [see footnote 1]). tine record we utilize here for uranium-series piedmont deposits were sampled and dated by dating has been interpreted to record pulses of optically stimulated luminescence (OSL) and

Faulting and Travertine CO2 leakage linked to climate changes (Kamp- uranium-series dating. OSL samples date the man et al., 2012). timing of sediment deposition and burial dur- The S-dipping Little Grand Wash normal Typical travertine along the Little Grand ing the episode of base-level stability and lateral fault has an arcuate surface trace of 61 km and Wash fault consists of centimeter-thick to tens- planation marked by the given strath terrace. The a total vertical separation in the study area of of-centimeters-thick, subhorizontal veins of radi- uranium-series results reported here are a small 180–210 m (Fig. 1A; Dockrill and Shipton, ating acicular calcite and aragonite crystals that subset of those reported in Burnside (2010), 2010). Like the other NW-SE–oriented normal have botryoidal or mammilated top surfaces. The specifi cally, travertine samples that underlie and faults of the region, the Little Grand Wash fault A.D. 1867 Powell expedition documented “satin interfi nger with piedmont-terrace alluvium or is presumed to sole in the Paradox Formation spar” at this location (Powell, 1875), which may that relate to Holocene fl oodplain deposits adja- evaporites at depth, though a deeper link to be these ray-crystal calcite veins. These veins cent to Crystal Geyser. These samples are not basement structures is also possible (Black and have been interpreted to generally form under of secondary cement, and results are interpreted Hecker, 1999; Trudgill, 2011). Previous regional saturated conditions, though occasionally they as ages of crystallization for the travertine and mapping documented river terraces overlying contain stalactite-like structures suggesting dis- therefore help constrain the age of the interfi n- and obscuring the trace of the fault (Doelling, solution and reprecipitation of calcite within gering and overlying fl uvial gravels (Table 1). 2002), implying little or no slip over an unde- open cavities above the water table (Dockrill, Rare lenses of fi ne- to coarse-grained, cross- termined amount of Quaternary time. Indeed, 2006; Gratier et al., 2012). Ancient travertine bedded, fl uvial sand interbedded within other- the Little Grand Wash fault is not included in deposits along the fault are found up to 37 m wise cobbly terrace deposits were targeted for the U.S. Geological Survey and Utah Geologi- above those presently forming at Crystal Geyser, OSL dating. Samples were collected in alumi- cal Survey’s listing of active Quaternary faults and they tend to form resistant caps to erosional num tubes, with depth, elevation, and latitude/ (Black and Hecker, 1999). On the other hand, remnants. An important factor for this study is a longitude noted for calculation of cosmic con- several neighboring and analogous faults such key travertine body just east of the Green River tribution to dose rate. Representative samples as the Salt Wash Graben faults immediately to that issues from one such remnant mound, and for the determination of water content and the south and the associated Salt Valley Gra- which has been dated using uranium-series dat- radiometric dose rate were collected, with sedi- ben and Moab faults to the east and southeast ing. This travertine stratigraphically underlies ment for dose rate sampled within the relatively are considered active (Fig. 1A; Hecker, 1993). and interfi ngers with tributary/piedmont allu- homogeneous sand lenses and taken systemati- Finally, based upon initial mapping of traver- vium in a complex terrace remnant graded to a cally within 20 cm surrounding the OSL sam-

83.9 ± 10.6 ka 1300 99.4 ± 12.4 ka 1290 P6/5 114-103 ka 96.8 ± 16.5 ka 1280 M6 1270 87.3 ± 11.2 ka Green River ca. 60 ka M6y

M5 ult 1260 P6/5 fa 41.8 ± 6.0 ka Crystal M4 1250 Geyser elevation (m) LGW 1240 M3 M2 Cretaceous Mancos Fm Jurassic Summerville Fm 1230 M1 V.E. = 5X Holocene 1220 Figure 2. Cross-sectional proﬁ le of the terrace stratigraphy at Crystal Geyser, looking downstream, with age results from optically stimulated luminescence (OSL) and uranium-series dating (Table 1). Y-axis is surveyed elevation of terrace treads and basal straths above the modern base-ﬂ ow stage of the Green River, whereas the relative width of deposits is schematic, representing their lateral extent in the landscape. M— main-stem Green River strath terraces, P—deposits of local piedmont systems graded to main-stem terraces. The M6y deposit’s undeformed and capping relation to the Little Grand Wash (LGW) fault is shown in Figure 3. V.E.—vertical exaggeration.

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TABLE 1. CRYSTAL GEYSER GEOCHRONOLOGY SUMMARY OSL sample Deposit* Depth (m) No. aliquots Equivalent dose (Gy) (overdispersion) Dose rate (Gy/k.y.) OSL age (ka)† USU-271 M3 3.0 27 (35) 118.03 ± 12.12 (20.1%) 2.83 ± 0.16 41.8 ± 6.0 USU-278 M4 1.7 27 (43) 145.72 ± 15.31 (23.4%) 2.47§ ~60§ USU-256 P6/5 4.0 34 (45) 187.39 ± 15.03 (19.4%) 2.15 ± 0.12 87.3 ± 11.2 USU-279 P6/5 2.5 25 (45) 222.81 ± 17.45 (16.4%) 2.66 ± 0.14 83.9 ± 10.6 USU-780 M6 2.5 24 (48) 232.27 ± 32.20 (28.1%) 2.40 ± 0.13 96.8 ± 16.5 USU-781 M6 1.9 27 (41) 227.79 ± 17.27 (14.9%) 2.29 ± 0.12 99.4 ± 12.4 U-series sample Deposit Total U (ppm) Total Th (ppb) 230Th/232Th Age (ka)# LG.03.42AZ P6/5 4.754 – – 103.2 ± 1.5 LG.03.42AX P6/5 5.093 0.38 888,662 106.5 ± 0.5 LG.03.42AF P6/5 4.220 0.06 675,748 109.6 ± 0.9 LG.03.42AD P6/5 5.076 0.04 905,311 113.9 ± 0.6 Note: OSL—optically stimulated luminescence. *Organized by stratigraphic position; M—main-stem Green River, P—local piedmont drainages graded to main stem. †Reported ages are at 2σ with random and systematic errors combined in quadrature. §Mean dose rate of other samples used due to erroneously high chemistry results (see text and Table DR2 [see text footnote 1]); age is only an estimate. #U-series ages are at 2σ, with systematic analytical error and error on decay constants propagated into age-error calculation.

ples. For radiogenic dose rate, the bulk sediment sample. The range of age results is interpreted tion of the M6, capping P6 (“piedmont-6”) and concentrations of K, Rb, U, and Th were mea- to refl ect the duration of the episode of spring P5, and the inset M3 terraces (Table 1). When sured using inductively coupled plasma–atomic activity recorded in the deposit. combined with the uranium-series results from emission spectrometry (ICP-AES) and ICP mass the basal P6 deposit, it is evident that the M6, spectrometry (MS) techniques. Total dose rates RESULTS M6y, and M5 terraces, which dominate the were calculated using the methods of Aitken local landscape, represent a complex episode (1998) and Prescott and Hutton (1994). OSL Chronostratigraphy of river planation and gravel deposition from measurements were conducted at the Utah State ca. 115 to 85 ka (Fig. 2). This correlates well to University Luminescence Laboratory on a quartz Seven distinct main-stem (M1–M7) Green a similarly prominent terrace that is well dated fraction ranging within 75–250 μm using a RISO River strath-terrace deposits were identifi ed in and recorded downstream in the Grand Canyon TL/OSL-DA-20 reader, following the single-ali- the Crystal Geyser map area (Fig. 1B), with area, where it is a thick fi ll terrace labeled M4 quot regenerative (SAR) protocol of Murray and some additional fi ll-cut terrace levels beveled (Pederson et al., 2013). Likewise, the ca. 42 ka Wintle (2000). Thirty-fi ve to 48 aliquots were upon them (DR1 map [see footnote 1]). A rela- M3 deposit at Crystal Geyser has a well-dated measured from each sample, with optical ages tively minor strath terrace labeled M6y (for analog along the Colorado River in both the calculated using the central-age model of Gal- “younger”) lies intermediate between the exten- neighboring Moab, Utah, area and in Grand braith et al. (1999) on those aliquots that passed sive M6 and M5 below, and this terrace has Canyon (Jochems, 2013; Pederson et al., 2013). standard criteria for quality and optical behavior. important relations, discussed in the following. These initial indications that episodes of river Detailed OSL laboratory methods, including The most prominent Pleistocene terraces range planation and sedimentation, with intervening such analytical criteria, and full equivalent-dose from 12 m (M2) to 56 m (M6) in height above incision, correlate at Milankovitch time scales distribution and dose-rate results for all samples the modern channel edge. The terrace treads across the region are consistent with terrace for- are given in Data Repository Item 2 (see footnote have poorly developed desert pavements and mation being broadly driven by the effects of 1). Total 2σ errors in Table 1 are combined ran- complex calcic and gypsic soils. M1 is a series climate change on sediment supply and trans- dom and systematic errors, including instrument of fi ner-grained Holocene deposits of the river port, rather than linked to different, specifi c calibration, uncertainty in dose-rate calculation, fl oodplain, and it includes associated travertine base-level changes propagating up the river sys- and equivalent-dose scatter. near Crystal Geyser with dates ranging from ca. tem (e.g., Bull, 1991; Hancock and Anderson, Aragonite samples were uranium-series 9 to 5 ka (Fig. 2; Burnside et al., 2013). Pleis- 2002; Finnegan and Balco, 2013). dated by mass spectrometer at the Scottish Uni- tocene terraces M2-–7 have well-developed and Between these well-dated terraces, the M4 versities Environmental Research Centre fol- exposed planar straths and are capped by up to terrace deposit has an OSL result that is less lowing the methods of Ellam and Keefe (2007). 10 m of cobbly alluvium where fully preserved reliable. Triplicate chemistry results confi rm an Sample processing to preconcentrate and sepa- (Figs. 2 and 3). The deposits are clast-supported, anomalously high environmental dose rate for rate U and Th is described in detail in Burnside rounded, and strongly imbricated, pebble-cob- this sample, specifi cally due to high concentra- (2010). Ages were calculated using Isoplot/Ex ble gravel with tabular to broadly lenticular, tions of uranium and thorium in the sediment rev. 2.49 (Ludwig, 2001) and the decay constants medium bedding, and rare sand lenses. Clasts and potential disequilibrium in the uranium- of Cheng et al. (2000). Final ages are reported in are dominated by sandstone transported from series decay chain relative to the other samples Table 1 with 2σ total analytical errors. To ensure Desolation Canyon and quartzite from the Uinta of Green River alluvium (DR2 [see footnote 1]). the reproducibility of travertine ages, three sam- Mountains over 300 km upstream. Finer-grained The initial age result is therefore anomalously ples from the greater study area were used for overland-fl ow deposits of local piedmont slopes young. The sand lens sampled in this terrace repeat analysis in Burnside et al. (2013). The interfi nger with and prograde over these distinc- gravel lies at a relatively shallow depth of 1.7 m, ages reported here from the travertine mound tive main-stem gravels (Fig. 2). within a clear gypsic B horizon of the soil pro- associated with the P6/5 deposit include white- Luminescence age results provide strati- fi le observed there. Gypsum is generally abun- banded vein samples and a single-layered mat graphically coherent age estimates for deposi- dant in the Mancos Shale parent material, which

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mate an overall bedrock incision rate integrated over the length of these climate-driven episodes (e.g., Bull, 1991; Pederson et al., 2006). Along the Green River at Crystal Geyser, this results in a net bedrock incision rate of ~450 m/m.y., which is the slope of the visually estimated trend line drawn in Figure 4 (a strict regression through data points is unjustiﬁ ed and would imply greater precision than exists). This result is similar to other surprisingly rapid trunk-river incision rates calculated in the central Colorado Plateau over the late Pleistocene and utilizing multiple age constraints (e.g., Marchetti and Cerling, 2005; Pederson et al., 2013). On the other hand, it is at least three times faster than well-constrained incision rates from Grand Can- yon farther downstream (Pederson et al., 2006).

Figure 3. Photograph looking west across a gully drainage into the trace of the Little Grand DISCUSSION Wash normal fault, here offsetting Cretaceous Cedar Mountain Formation (Kcm) against the Jurassic Summerville Formation (Js) in the footwall (hachures denote zone of fault defor- Fault Expression and Timing mation). The undeformed strath of the M6y terrace crosscuts the fault zone, indicating no measurable surface rupture over the past ~100 k.y. The Little Grand Wash fault is clearly expressed in the landscape of the study area, marked by an escarpment at Crystal Geyser. Yet, is also known for elevated concentrations of ura- mm/yr, it would produce 1 m of offset over the late Quaternary fault movement has not contrib- nium (Pliler and Adams, 1962). This particular 100 k.y. of our record and be reﬂ ected by fault- uted to this relief. This escarpment is, therefore, sample locality may have higher amounts of that line scarps and offset of Quaternary units. It is mostly the product of differential, and relatively parent material, and it is possible that uranium not, and we conclude that there has been no sur- rapid, erosion of the landscape—in this case has also translocated to the depth of the OSL face rupture of the fault over the past ~100 k.y. of the weak Mancos Shale relative to the more sample over the arid Holocene. Regardless, the Likewise, the travertine mounds along the resistant Summerville and Morrison sandstones modern dose rate measured is likely higher than fault trace straddle the fault without being offset, and mudstones outcropping in the hanging-wall the true time-averaged dose rate to which the and there is no offset of the internal layering of block. Such erosional relief from contrasting sample was subjected over its longer history of the travertine (Dockrill, 2006; Burnside, 2010). rock units juxtaposed along an inactive fault has burial. When we substitute the mean dose rate of Disrupted and rotated veins and layering within also been documented in the Sierra Nacimiento all other samples in this study, the resulting age the local travertine mounds have been inter- at the southeast edge of the Colorado Plateau estimate of ca. 60 ka is in stratigraphic order. preted as potential indicators of seismic activity (Shipton et al., 2004) and surface rupture (ﬁ g. 4f Field Evidence Regarding Fault Activity in Kampman et al., 2012). However, we suggest 80 that such textures are equally likely to be due to 450 m/my The Little Grand Wash fault is expressed in failure of wall rock in an eruption of groundwa- 60 M6 ? the landscape by an embayed escarpment held ter (Uysal et al., 2009), progressive erosion of M5 40 up by the relatively resistant Jurassic sandstones the base of the mounds that overlie the softer M4 along the footwall. There is no evidence for Mancos Shale (Burnside, 2010), or even crystal- M3 20 scarps along the actual fault trace running south lization processes (Gratier et al., 2012). Kamp- M2

of this escarpment, and piedmont slopes and man et al. (2012) hypothesized that periodic 0 ? river-terrace treads cross the fault without topo- changes in CO2 leakage from this fault relate

height above reference stage (m) 020 4060 80100 120 140 graphic defl ection. Field relations between the to fault dilation from groundwater and fl ex- time (ka) fault plane and the strath terraces of the study ural loading and unloading during Pleistocene area provide more direct evidence that there climate changes. If so, those mechanisms also Figure 4. Plot of reconstructed Green River grade through late Quaternary time relative to has been no recent measurable slip on the Little caused no vertical surface offset on the fault. modern base-fl ow elevation, as constrained by Grand Wash fault. The most visible example is the surveyed terrace chronostratigraphy illus- the undeformed basal strath of the M6y terrace Incision History trated in Figure 2. Circles are optically stimu- overlying the trace of the fault exposed in a gully lated luminescence (OSL) ages, with bars rep- drainage (Fig. 3). Although the M6y deposit is The age and surveyed geometry of fl uvial resenting reported error, and the two connected not directly dated, it is in a landscape position terraces can be used to reconstruct the episodic diamonds represent range of uranium-series results. The interpreted episodes of lateral pla- just 4 m below the larger M6, which is con- history of river planation and bed-load storage, nation and terrace deposition are separated by strained by both uranium-series and OSL ages separated by episodes of incision, over the past relatively rapid incision through weak bedrock to 115–95 ka. If the Little Grand Wash fault in ~100 k.y. (Fig. 4). This plot of channel posi- (gray). The rate of incision trending through this arid setting had even slow slip rates of 0.01 tion, or grade, through time enables us to esti- these cycles is ~450 m/m.y.

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(Formento-Trigilio and Pazzaglia, 1998). That tectonic extension could drive active deformation Instead, this bull’s-eye is interpreted to refl ect a study provided a general conclusion that is also in the region, although salt dissolution is more regionalized feedback between erosional exhu- pertinent in the central Colorado Plateau—the likely for the structures east of our study area mation and the fl exural-isostatic rebound men- characteristic high-relief topography is due more (Gutiérrez, 2004). Related to this regional exten- tioned earlier. Indeed, thermochronology results to base-level fall from recent drainage integra- sion, ~100 km to the southwest of the study area from just east of Crystal Geyser along the Book tion and differential erosion of varying bedrock in the east-central plateau, there are exhumed Cliffs and in the northern canyonlands confi rm than from active tectonics (Formento-Trigilio and mafi c dikes dated to 4 Ma by K-Ar methods (Del- over 3 km of exhumation in the past ~5 m.y. Pazzaglia, 1998; Pederson and Tressler, 2012). aney and Gartner, 1997). These have a similar (Hoffman et al., 2011). This feedback of erosion Constraints for the timing of slip on the Lit- average NW-SE orientation as the normal faults and rock uplift may drive enhanced incision in tle Grand Wash fault are limited, even with our of the region, and it is possible the Little Grand the central plateau as well as extension, but it new data. It was active sometime before 100 Wash and other faults were active at this same was ultimately initiated by the 5–6 Ma integra- ka, and it offsets late Cretaceous strata. Dock- time, responding to the extensional stress fi eld. tion and base-level fall of the Colorado River rill (2006) used a database of local boreholes Intriguingly, this possible Pliocene compo- propagating upstream from the plateau edge to confi rm that there are no resolvable thick- nent of extension and faulting in the region coin- west of Grand Canyon (e.g., Pederson et al., ness changes across the fault from the Permian cides with the start of rapid erosional unload- 2002; Dorsey et al., 2007). to the Cretaceous. He suggested that the fault ing in the central plateau (Hoffman et al., 2011; Rapid incision in the very broad-valley land- initiated during Early Tertiary Laramide tecto- Lee et al., 2013). In fact, it has been repeatedly scape around Crystal Geyser may seem contrary nism, which was prevalent across the region. A hypothesized that late Cenozoic base-level fall, to expectations at fi rst, in that there is typically proportion of slip on the Moab fault to the east localized erosion, and relief production have an intuitive correspondence among steep rivers, has been dated to the Early Tertiary (Pevear et driven diapiric movement of salt elsewhere in high canyon relief, rapid incision, and active al., 1997; Solum et al., 2005), supporting the the Paradox Basin through differential unload- uplift. Yet, local relief and tectonic activity are not possibility of Laramide movement on the Little ing (e.g., Cater, 1970; Colman, 1983; Trudgill, always coupled. At Crystal Geyser, the rapidly Grand Wash fault, yet a younger component of 2011). We suggest that broader-wavelength incising Green River has a relatively low gradient slip is also possible. unloading also may have resulted in movement, and low energy expenditure (Fig. 5; Pederson and The present-day inactivity of the Little Grand including along the Little Grand Wash fault and Tressler, 2012), and the entire Labyrinth Canyon Wash fault contrasts with the active deformation Salt Wash Graben. The modeled pattern of iso- just downstream lacks any named rapids. Below of related structures across the ancestral Paradox static rebound from late Cenozoic exhumation Cataract Canyon, the Colorado River is likewise Basin, particularly to the southeast of the study is roughly domal, peaking in the central plateau low in gradient, yet it has high incision rates like area in the Needles fault zone of Canyonlands and diminishing proportionally to its edges (Cal- that at Crystal Geyser (Fig. 5; Pederson et al., National Park and to the east across the Utah- lahan et al., 2006; Karlstrom et al., 2012; Lazear 2013). These low-energy, central plateau reaches Colorado border region. The northern Paradox et al., 2013). This arching pattern of enhanced coincide with a sequence of Jurassic and Creta- Basin is deformed into a series of roughly paral- rock uplift across the plateau would enhance ceous shales and sandstones at river level that lel NW-trending faults and salt-cored folds. Dis- extension and could have inspired slip along are of low strength and that provide little coarse solution of Pennsylvanian salt by groundwater, regional normal faults, accommodated by salt bed material to the channel. Despite this, abun- and then removal by the Colorado River system, motion at depth. This is analogous to the effects dant tools for river incision do exist, having been is cited as the driver of ongoing graben subsid- of similarly broad and domal rebound in other transported from Desolation Canyon and farther ence along the crests of the anticlines (Colman, regions due to glacial unloading (Muir-Wood, upstream, as evident from the gravel-clast types 1983; Huntoon, 1988; Gutiérrez, 2004; Trudgill, 2000). This new hypothesis links a component of the terrace alluvium in the study area. Hard 2011). In fact, one of these diapiric salt-cored of fault deformation to the broader regional his- tools on soft rock accomplish signifi cant incision, folds is the Green River anticline, and the Crys- tory of erosion and incision. as well as lateral planation, forming prominent tal Geyser well was drilled where this N-plung- strath terraces (e.g., Montgomery, 2004; Johnson ing anticline is cut almost orthogonally by the Regional Patterns of River Incision et al., 2009). Overall, the Green River here has a Little Grand Wash fault (Fig. 1). Yet, the Para- channel gradient set by a relatively thin mantle of dox Formation evaporites are stratigraphically The river incision rate of 450 m/m.y. at bed load in transport, which, although gentle, is >1.5 km below the surface in this study area, and Crystal Geyser over late Quaternary time is more than adequate to achieve the required inci- it is unclear from drilling records whether the three times faster than the typical western inte- sion rate through weak bedrock. fault cuts the evaporites (Shipton et al., 2004). rior U.S. average of 150 m/m.y. (Dethier, 2001), Upstream in the Desolation Canyon knick Thus, we suggest that groundwater dissolution but it is consistent with the recently recognized zone, the river steepens where it crosses the Book and salt tectonics have not been the main drivers bull’s-eye pattern of rapid incision in the central and Roan Cliffs (Fig. 5). Despite this steeper gra- for deformation (or the lack thereof) along the Colorado Plateau (Pederson et al., 2013). Given dient and the deep canyon, there is evidence that Little Grand Wash fault. that this relatively rapid incision is not linked to incision rates actually may be much slower than Any post-Laramide slip on the Little Grand active local faulting, broader sources of base- at Crystal Geyser. Darling et al. (2012) reported Wash and nearby Salt Wash Graben faults may level fall are required. In terms of active uplift, an incision rate a full order of magnitude slower relate instead to the overall SW-NE extensional Pederson et al. (2013) pointed out that fast inci- (43 m/m.y.) at the mouth of Tabyago Canyon state of stress in the central Colorado Plateau, sion here in the central plateau is inconsistent near the head of Desolation Canyon (Fig. 5), which is linked to Basin and Range rifting along with recently proposed mantle-driven uplift based upon an isochron cosmogenic-burial age the plateau margins (Wong and Humphrey, at the SW fl ank of the plateau and associated of 1.48 Ma on a terrace deposit in a similar land- 1989). Indeed, based upon physical modeling downward tilting of the central and NE plateau scape position as the M6 terrace at Crystal Gey- of salt-related structures in the ancestral Paradox (Moucha et al., 2009; van Wijk et al., 2010; ser. This is only a single age result based upon Basin, Ge and Jackson (1998) suggested that this Levander et al., 2011; Karlstrom et al., 2012). three clasts in that terrace deposit, and poten-

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Crystal Geyser Uinta Basin A Central Plateau Province 2000 Grand Canyon Province 1600 Province 1600 1200 1200 800 confluence of Tabyago Green and Colorado Canyon elevation (m) 400 100x v.e. 180 120 60 0 -60 -120 -180 -240 -300 -360 -420 -480 -540 B river mile ) 2 500 500 u Ω 0 0 (watts/m Grand Canyon Cataract Desolation knick zone knick zone knick zone C

0 0 (m/m.y.) ? 200 ? 200 400 400 incision rate

Figure 5. (A) Longitudinal profi le of the Green-Colorado River system starting where the Green River leaves the Uinta Mountains and crosses the Uinta Basin, through the Desolation knick zone, and past Crystal Geyser to the confl uence of the Green and Colorado Rivers at the head of Cataract Canyon, and then through Glen Canyon and fi nally the Grand Canyon knick zone. Tick marks bound reaches defi ned by bedrock substrate. (B) Reach-averaged unit stream power from Pederson and Tressler (2012). (C) Trend of comparably calculated late Quaternary river incision rates (larger black dots) from Pederson et al. (2013), with the white-centered data point being the result for Crystal Geyser. Note the lack of correspondence between the broad-wavelength pattern of incision rate and shorter variations in river steepness or energy, with the most rapid regional incision occurring in broad, low-energy reaches like Crystal Geyser. The potential decrease in incision rate through the Desolation knick zone is suggested by the longer-time-scale results of Darling et al. (2012) at Tabyago Canyon (smaller black dot), consistent with the interpretation that the knick zone potentially refl ects a transient wave of incision in the system. V.E.—vertical exaggeration.

tial sources of error in this cosmogenic-dating Crystal Geyser over the past 100 k.y. may rep- Cheng, H., Edwards, R.L., Hoff, J., Gallup, C.D., Richards, D.A., approach include the fact that those clasts may resent just a steady snapshot within a longer, and Asmerom, Y., 2000, The halfl ives of uranium-234 and thorium-230: Chemical Geology, v. 169, no. 1–2, p. 17–33, share a signifi cant previous history of episodic complexly changing and linked late Cenozoic doi:10.1016/S0009-2541(99)00157-6. burial and transport. Furthermore, this low inci- history of erosion and deformation. Colman, S.M., 1983, Infl uence of the Onion Creek salt diapir on the late Cenozoic history of Fisher Valley, southeastern sion rate was calculated over a much longer time Utah: Geology, v. 11, no. 4, p. 240–243, doi:10.1130/0091- span than ours at Crystal Geyser, and therefore ACKNOWLEDGMENTS 7613(1983)11<240:IOTOCS>2.0.CO;2. the difference may be partly due to averaging We thank Michelle Nelson at the Utah State University Lumi- Colman, S.M., and Hawkins, F.F., 1985, Surfi cial Geologic Map nescence Laboratory for her work on optically stimulated of the Fisher Valley–Professor Valley Area, Southeastern rates over longer variations or hiatuses in pro- luminescence samples and Dr. Janis Boettinger at Utah State Utah: U.S. Geological Survey Miscellaneous Investiga- cesses (Gardner et al., 1987). University for her knowledge of soils in the study area. This tions Series Map I-1596, scale 1:24,000, 1 sheet. manuscript was improved by three anonymous reviews. Recognizing the contrast between their Cook, K.L., Whipple, K.X., Heimsath, A.M., and Hanks, T.C., Burnside was supported by UK Natural Environment Research 2009, Rapid incision of the Colorado River in Grand Can- older burial ages and other younger terrace Council grant NER/S/A/2006/14354. yon; insights from channel profi les, local incision rates, chronostratigraphies in the region, Darling et and modeling of lithologic controls: Earth Surface Pro- al. (2012) suggested that river incision across REFERENCES CITED cesses and Landforms, v. 34, p. 994–1010. Aitken, M.J., 1998, An Introduction to Optical Dating: Oxford, Darling, A.L., Karlstrom, K.E., Granger, D.E., Aslan, A., Kirby, the region may have greatly increased in rate UK, Oxford Science Publications, 280 p. E., Ouimet, W.B., Lazear, G.D., Coblentz, D.D., and Cole, at some point in the Pleistocene. 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Cater, F., 1970, Geology of the Salt Anticline Region in South- leakage from a natural CO2 geologic storage site: central 2007; Cook et al., 2009). In summary, the high western Colorado: U.S. Geological Survey Professional Utah, U.S.A.: Journal of Structural Geology, v. 32, no. 11, incision rate and inactive faulting we record at Paper 637, 80 p. p. 1768–1782, doi:10.1016/j.jsg.2010.01.007.

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